Year 424 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Tribunate of Crassus, Fidenas, Rutilus and Iullus (or, less frequently, year 330 Ab urbe condita). The denomination 424 BC for this year has been used since the early medieval period, when the Anno Dominicalendar era became the prevalent method in Europe for naming years.

At the Congress of Gela, the statesman Hermocrates of Syracuse persuades the cities of Sicily to agree to make peace and urges the exclusion of foreign powers. As a result, the three-year war between his city and Sicily's pro-Athenian towns ends and the Athenian forces, which had been sent to Sicily to support Greek settlements, are forced to withdraw.

After he frustrates the Athenian attack on Megara, Brasidas marches through Boeotia and Thessaly to Chalcidice at the head of 700 helots and 1000 Peloponnesian mercenaries to join the Macedonian king Perdiccas II. Refusing to be made a tool for the furtherance of Perdiccas' ambitions, Brasidas wins over the important cities of Acanthus, Stagirus, Amphipolis and Torone as well as a number of minor towns. An attack on Eion is foiled by the arrival of Thucydides at the head of an Athenian squadron.

Brasidas' capture of the city of Amphipolis is a major reverse for Athens, for which the Athenian general (and future historian) Thucydides is held responsible and banished. This gives Thucydides the opportunity for undistracted study for his History and for travel and wider contacts, especially on the Peloponnesian side (Sparta and its allies).

Nicias captures the Peloponnesian island of Cythera, from which to harry the Spartans.